1. Field of the Invention
This invention relates to a reflector type multibeam antenna and a method of configuring the antenna.
2. Description of the Prior Art
There are prior art multi-beam antennas composed of several reflectors such as (1) a uni-focal antenna: e.g., an offset paraboloid antenna, and an offset cassegrain antenna and (2) a bifocal antenna.
The former, or the uni-focal antenna of (2), has two foci: one in the vicinity of the reflector and the other at an infinite distance therefrom and is available as a high gain single beam antenna.
The latter, or the bifocal antenna of (2), consists of a proper arrangement of a main reflector and sub-reflectors, having four foci: two near the reflectors and the other two far from them. As the bifocal antenna can radiate at least two high performance beams, it is better than the antenna of (1) in principle.
An antenna having the aforementioned foci has the characteristic that the phase error at its aperture surface is proportional to the amount of deviation when the antenna is fed at a point deviated from the foci. Because of this characteristic, the performance of the beam radiation (e.g., gain and side-lobe characteristics) becomes worse at increasing beam direction angles relative to the direction of the focus at the infinite distance.
FIG. 1(a) shows a uni-focal antenna radiation pattern in case of offset feeding. The abscissa of FIG. 1(a) is the beam direction angle .theta. in the direction of the infinite distant focus, and the ordinate represents relative power.
In the figure, .theta.=0 implies the front end direction of the antenna where the peak value is maximum and the side-lobes are small.
In a uni-focal antenna, the direction represented by .theta.=0 is in agreement with the direction of focus in infinite distance. At angle .theta..sub.1, the peak value is smaller and the sidelobes are larger than those at .theta.=0. The dotted line of the FIG. 1(a) represents an envelope of the peak values.
FIG. 1(b) shows the contours of the envelope represented in FIG. 1(a).
FIG. 2(a) shows the radiation patterns of a bifocal antenna having offset feeding. In the figure, .theta.=0 implies the front end direction of the antenna, and .theta.=.+-..theta..sub.0 represents the direction of focus in the infinite distance. The dotted line in FIG. 2(a) shows the peak envelope, and FIG. 2(b) shows the contours of the envelope shown in FIG. 2(a).
From FIGS. 1 and 2, it is apparent that the performance of the radiation beam is deteriorated as the angle .theta. of beam direction with infinite focus direction increases. As the uni-focal antenna and the bifocal antenna have the characteristics mentioned above, a multi-beam antenna provided with three or more feeder horns in front of the main reflector of either of the above mentioned types of antenna may generate one or more poor performance beams. To get a multi-beam antenna free from this inconvenience, some attempts such as adjusting the phase of the poor performance beams have been made.
However, the disadvantages of such a phase adjusting method is that it is time consuming, troublesome work. In addition, requires the use of electric component parts such as a phase shifter, which pushes the cost up.